Shallow-water hydrothermal vents are extreme environments that share many characteristics with their deep-sea analogs. However, despite ease of access, much less is known about the geochemical dynamics of these ecosystems. Here, we report on the spatial and temporal geochemical dynamics of a shallow-water vent system at Paleochori Bay, Milos Island, Greece. Our multi-analyte voltammetric profiles of dissolved O-2 and hydrothermal tracers (e.g. Fe2+, FeSaq, Mn2+) on sediment cores taken along a transection in hydrothermally affected sediments indicate three different areas: the central vent area (highest temperature) with a deeper penetration of oxygen into the sediment, and a lack of dissolved Fe2+ and Mn2+; a middle area (0.5 m away) rich in dissolved Fe2+ and Mn2+ (exceeding 2 mM) and high free sulfide with potential for microbial sulfide oxidation as suggested by the presence of white mats at the sediment surface; and, finally, an outer rim area (1-1.5 m away) with lower concentrations of Fe2+ and Mn2+ and higher signals of FeSaq, indicating an aged hydrothermal fluid contribution. In addition, high-frequency temperature series and continuous in situ H2S measurements with voltammetric sensors over a 6-day time period at a distance 0.5 m away from the vent center showed substantial variability in temperature (31.6 to 46.4 degrees C) and total sulfide (488 to 1329 mu M) in the upper sediment layer. The analysis of these data suggests that tidal and wind forcing, and abrupt geodynamic events generate intermittent mixing conditions lasting for several hours to days. Despite substantial variability, the concentration of sulfide available for chemoautotrophic microbes remained high. However, the availability of electron acceptors originating from seawater might be more intermittent, which in turn has an effect on the reestablishment of the white mats after wave-induced disturbances. Our results emphasize the importance of transient events in the development of chemoautotrophic communities in the hydrothermally influenced sediments of Paleochori Bay. (C) 2013 Elsevier B.V. All rights reserved.

Themobility ofmetals in soils and subsurface aquifers is strongly affected by sorption and complexation with dissolved organic matter, oxyhydroxides, clay minerals, and inorganic ligands. Humic substances (HS) are organic macromolecules with functional groups that have a strong affinity for binding metals, such as actinides. Thorium, often studied as an analog for tetravalent actinides, has also been shown to strongly associate with dissolved and colloidal HS in natural waters. The effects of HS on the mobilization dynamics of actinides are of particular interest in risk assessment of nuclear waste repositories.

Natural hydrothermal vent environments cover a wide range of physicochemical conditions involving temperature, pH and redox state. The stability of simple biomolecules such as amino acids in such environments is of interest in various fields of study from the origin of life to the metabolism of microbes at the present day. Numerous previous experimental studies have suggested that amino acids are unstable under hydrothermal conditions and decompose rapidly. However, previous studies have not effectively controlled the redox state of the hydrothermal fluids. Here we studied the stability of glutamate with and without reducing hydrothermal conditions imposed by 13 mM aqueous H-2 at temperatures of 150, 200 and 250 degrees C and initial (25 degrees C) pH values of 6 and 10 in a flow-through hydrothermal reactor with reaction times from 3 to 36 min. We combined the experimental measurements with theoretical calculations to model the in situ aqueous speciation and pH values. As previously observed under hydrothermal conditions, the main reaction involves glutamate cyclizing to pyroglutamate through a simple dehydration reaction. However, the amounts of decomposition products of the glutamate detected, including succinate, formate, carbon dioxide and ammonia depend on the temperature, the pH and particularly the redox state of the fluid. In the absence of dissolved H-2, glutamate decomposes in the sequence glutamate, glutaconate, alpha-hydroxyglutarate, ketoglutarate, formate and succinate, and ultimately to CO2 and micromolar quantities of H-2(aq). Model speciation calculations indicate the CO2, formate and H-2(aq) are not in metastable thermodynamic equilibrium. However, with 13 mM H-2(aq) concentrations, the amounts of decomposition products are suppressed at all temperatures and pH values investigated. The small amounts of CO2 and formate present are calculated to be in metastable equilibrium with the H-2. It is further proposed that there is a metastable equilibrium between glutamate, glutaconate, alpha-hydroxyglutarate, ketoglutarate and H-2. The key redox-sensitive step is the reaction of alpha-hydroxyglutarate to alpha-ketoglutarate, which is effectively inhibited by the elevated H-2 concentrations, which in turn dramatically lowers the amounts of all decomposition products including ammonia. Theoretical calculations of the metastable thermodynamic equilibrium between glutamate and ketoglutarate are consistent with the experimentally determined effects of reducing conditions. These findings establish that redox state is as important a variable as temperature and pH in affecting the stability of amino acids under hydrothermal conditions. It is suggested that when natural hydrothermal fluids contain enough dissolved H-2, the stability of amino acids may be enhanced in fluids at least on short time scales. In turn, this result suggests that reducing hydrothermal environments may have been favorable for assembling the building blocks of biomolecules in the origin of life. Furthermore, in present day hydrothermal vents the microbial ecosystems may in part be supported by the availability of metastable amino acids through heterotrophic metabolism. Published by Elsevier Ltd.

The potential for chemical evolution of complex organic molecules such as peptides in hydrothermal environments requires the persistence of the component amino acids under such conditions. Here, we show experimentally that the redox state (activity of H-2) of the aqueous fluids plays a key role in the stability of glutamic acid during hydrothermal processes. The results demonstrate that highly reducing redox conditions imposed by elevated concentrations of dissolved H-2 suppresses the oxidative decomposition of glutamic acid at elevated temperatures. Our experimental data support proposals that amino acids may persist, albeit metastably, under geochemically relevant hydrothermal conditions. The reduced nature of deep-sea vent fluids might have been a critical parameter in sustaining the needed ingredients for the origin of life on the early Earth, and may currently play a role in facilitating the persistence of biomolecules supporting heterotrophic microbial communities in modern near-seafloor hydrothermal environments. Published by Elsevier B.V.

The effect of pressure and temperature on the structure of silicate melts coexisting with silica-saturated aqueous electrolyte fluids enriched in fluorine or chlorine in the Na2O-Al2O3-SiO2-H2O system has been described. In situ measurements were conducted with the samples at desired temperatures and pressures in a hydrothermal diamond-anvil cell (HDAC) by using microRaman and FTIR spectroscopy techniques. The data were acquired at temperatures and pressures up to 800 degrees C and 1264 MPa, respectively.

The structure of water-saturated Ca- and Mg-bearing carbonate melts under reducing and oxidizing conditions was investigated in a series of hydrothermal anvil cell experiments conducted at 400-1100 degrees C and 442-2839 MPa. Equilibria were investigated in the calcite-H2O, calcite-CaO-H2O, magnesite-H2O, and magnesite-MgO-H2O systems, with redox conditions controlled by Re/ReO2 and Ti/TiO2 assemblages. Melting relationships and the C-O-H speciation of the coexisting aqueous fluid and melt were assessed in situ by Raman vibrational spectroscopy. Hydrous melting of MgCO3-MgO occurred at similar to 850 degrees C, 1.5-2 GPa. In the CaCO3-CaO-H2O system, melt was formed at 600-900 degrees C and pressures of 0.5-1.5 GPa because of melting-point depression imposed by the presence of CaO. The C-O-H speciation of the carbonate melts and coexisting supercritical aqueous solutions was mainly H2O and CO32-, with traces of CO2(aq) and CH4(aq) in the fluid phase. The melt-fluid H2O partition coefficients attained in the Mg-bearing melt (median 0.5) were higher than in the Ca-bearing melt (median 0.3). Under oxidizing redox conditions, dissolved ReO2- was present in all phases, underscoring the enhanced solubility of metals in carbonate-bearing melts and carbonatites. In effect, the enhanced solubility of H2O along with the ionic nature of the carbonate melts may promote the solvation of ionic species in the melt structure.

Awaruite (Ni2Fe to Ni3Fe) is often used to infer fugacity and redox gradients in hydrothermally altered peridotites. However, discrepant petrological and thermodynamic data suggest that the fO(2)-fS(2) stability field of awaruite is not well constrained. In this study, we assess the thermodynamic properties of awaruite and re-evaluate the Fe-Ni-S systematics of hydrothermally altered peridotites. New experimental data indicate that awaruite is stable at higher fO(2) than previously thought, supporting the common occurrence of awaruite in the reaction zone of modern and ancient ultramafic-hosted hydrothermal vent systems. Awaruite is known to catalyze the abiogenic synthesis of methane during active serpentinization, contributing to methanogenesis at modern oceanic hydrothermal systems and potentially on early Earth. The enhanced stability field of awaruite determined here suggests that abiogenic methanogenesis may be active at a broad range of redox conditions.

Hydrated (with D2O and H2O) sodium tetrasilicate glasses, quenched from melts at 1400 degrees C and 1.5 GPa, are studied using H-1, H-2, and Si-29 solid-state nuclear magnetic resonance (NMR) spectroscopy. Whereas D2O and H2O depolymerize the silicate melt to similar degrees, protium and deuterium intramolecular partitioning between different molecular sites within the glasses is very different and exemplified by a strong preferential association of deuterons to sites with short O-D center dot center dot center dot O distances. This preference is independent of total water content and D/H ratio. Substantially different intramolecular D-H partitioning is also observed in a glass with a model hydrous basalt composition. Such large differences in isotope partitioning cannot result from classic equilibrium fractionation because of the high synthesis temperature. Potential kinetic isotope effects are excluded via a slow quench experiment. The apparent fractionation is likely governed by density/molar volume isotope effects, where deuterium prefers sites with smaller molar volume. Large differences in intramolecular site partitioning in melts could lead to significant differences in D-H partitioning between water-saturated melt and exsolved aqueous fluid (where D/H-W,(Melt) not equal D/H-W,(Fluid)) during crystallization of Earth's magma ocean, potentially controlling the D/H content of the Earth's oceans.

Continuous culture under elevated pressures is an important technique for expanding the exploration of microbial growth and survival in extreme environments associated with the deep biosphere. Here we present a benchtop stirred continuous culture bioreactor capable of withstanding temperatures ranging from 25 to 120 degrees C and pressures as high as 69 MPa. The system is configured to allow the employment of media enriched in dissolved gases, under oxic or anoxic conditions, while permitting periodic sampling of the incubated organisms with minimal physical/chemical disturbance inside the reactor. In a pilot experiment, the fermentative growth of the thermopiezophilic bacterium Marinitoga piezophila was investigated continuously for 382 h at 65 degrees C and at pressures ranging from 0.1 to 40 MPa while the medium flow rate was varied from 2 to 0.025 ml/min. The enhanced growth observed at 30 and 40 MPa and 0.025 ml/min supports the pressure preferences of M. piezophila when grown fermentatively. This assay successfully demonstrates the capabilities of the bioreactor for continuous culturing at a variety of dilution rates, pressures, and temperatures. We anticipate that this technology will accelerate our understanding of the physiological and metabolic status of microorganisms under temperature, pressure, and energy regimes resembling those of the Earth's piezosphere.

A novel thermophilic, anaerobic, mixotrophic bacterium, designated strain MAG-PB1(T), was isolated from a shallow-water hydrothermal vent system in Palaeochori Bay off the coast of the island of Milos, Greece. The cells were Gram-negative, rugose, short rods, approximately 1.0 mu m long and 0.5 mu m wide. Strain MAG-PB1(T) grew at 30-70 degrees C (optimum 60 degrees C), 0-50 g NaCl l(-1) (optimum 15-20 g l(-1)) and pH 5.5-8.0 (optimum pH 6.0). Generation time under optimal conditions was 2.5 h. Optimal growth occurred under chemolithoautotrophic conditions with H-2 as the energy source and CO2 as the carbon source. Fe(III), Mn(IV), arsenate and selenate were used as electron acceptors. Peptone, tryptone, Casamino acids, sucrose, yeast extract, D-fructose, alpha-D-glucose and (-)-D-arabinose also served as electron donors. No growth occurred in the presence of lactate or formate. The G+C content of the genomic DNA was 66.7 mol%. Phylogenetic analysis of the 16S rRNA gene sequence indicated that this organism is closely related to Deferrisoma camini, the first species of a recently described genus in the Deltaproteobacteria. Based on the 16S rRNA gene phylogenetic analysis and on physiological, biochemical and structural characteristics, the strain was found to represent a novel species, for which the name Deferrisoma palaeochoriense sp. nov. is proposed. The type strain is MAG-PB1(T) (=JCM 30394(T)=DSM 29363(T)).